Wrong attribut name in qt/worker.py

Got the following traceback when trying to load an account:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/accounts.py", line 176, in __relogin_account
    self.base.load_account(account_id)
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/main.py", line 366, in load_account
    self.core.load_account(account_id)
  File "/usr/lib/python2.7/site-packages/turpial/ui/qt/worker.py", line 339, in load_account
    self.register(self.core.accman.load, (account_id),
AttributeError: Core instance has no attribute 'accman'

This commit fixes it.